Extraction temperature and time are among the most critical considerations when brewing coffee. Automatic drip coffee makers (ADCs) brew better coffee than percolators by avoiding re-boiling coffee and reducing extraction time, thus preserving the aroma and reducing coffee bitterness. (The bitter, less soluble chemicals in the grounds require longer extraction time.) Shorter extraction time, however, normally causes incomplete extraction, which contributed to the weaker coffee made by single-cup ADCs and by displacement-type coffee makers (U.S. Pat. No. 5,200,221).
It is known that the best brewing temperature for coffee is 192° to 205° F. Such high temperature is easily achievable by boiling water in a percolator. But it is more difficult for ADCs, especially when a single cup of coffee is to be brewed, which also contributed to the weaker coffee made by single-cup ADCs.
In the applicant's earlier U.S. Pat. No. 5,267,506, apparatus is disclosed to brew coffee by extracting the grounds under elevated pressure to reduce the extraction time while achieving full flavor extraction. The apparatus has a cell in contact with a heater that draws an amount of water from a reservoir and subsequently heats the water and generates pressurized steam to deliver the hot water. In a first embodiment, the hot water in the cell is delivered to and through the flavor-containing materials under steam pressure and the steam is then released through a steam release valve. In a second embodiment, the hot water in the cell is first delivered and the steam is later delivered to the flavor-containing materials.
In the applicant's another earlier U.S. Pat. No. 6,405,637, apparatus and method are disclosed to brew coffee by generating and delivering a plurality of pressure pulses, each of which contains a pressurized hot water pulse and a pressurized steam pulse, to and through the flavor-containing materials to extract the useful compounds under the pressure of the pressure pulses.
Such apparatus have numerous advantages including higher brewing speed, less flavor loss and lower cost over known coffee makers and espresso machines. However, significant advancements can still be made with such apparatus to improve the taste and brewing time as well as the ease of use and cost of the apparatus.